Synthesis of large-sized spherical Co-C alloys with soft magnetic properties though a high-pressure solid-state metathesis reaction

In this work, we report a novel high-pressure solid-state metathesis (HSM) reaction to produce spherical bulk (diameters 2-4 mm) Co-C alloys (Co₃C and Co_1−xC_x). At 2-5 GPa and 1300 °C, C atoms preferentially occupy the interstitial sites of the face-centered cubic (fcc) Co lattice, leading to the formation of metastable Pnma Co₃C. The Co₃C decomposes above 1400 °C at 2-5 GPa, C atoms infiltrate the interstitial sites of the fcc Co lattice, saturating the C content in Co, forming an fcc Co_1−xC_x solid solution while the C atoms in excess are found to precipitate in the form of graphite. The Vickers hardness of the Co-C alloys is approximately 6.1 GPa, representing a 19.6% increase compared to hexagonal close-packed (hcp) Co. First-principles calculations indicate that the presence of C atoms in the Pnma Co₃C structure leads to a relative decrease in the magnetic moments of the two distinct Co atom occupancies. The Co-C alloys exhibited a soft magnetic behavior with saturation magnetization up to 93.71 emu g⁻¹ and coercivity of 74.8 Oe; coercivity increased as the synthesis pressure rises.